Engines may operate on gaseous fuel, such as natural gas. Both liquid natural gas (LNG) and gaseous, compressed natural gas (CNG) are being used as automotive applications. For CNG, the gas may be compressed and stored in cylinders under high pressure, utilizing a pressure regulating valve to supply the fuel to the engine combustion chamber at a lowered pressure. CNG fuels may be used with various engine systems, including mono-fuel systems that utilize CNG as the sole fuel source, and multi-fuel systems configured to utilize CNG alongside one or more additional fuels, including liquid fuels such as gasoline, diesel, or gasoline-ethanol blends.
Liquid fuel systems are subject to requirements regarding the prevention and detection of fuel vapor loss. This is referred to as evaporative emissions controls. While these systems have been extensively developed for mixed phase (liquid and vapor) fuels, they have not been developed for gaseous fuels such as CNG.
Evaporative emissions may be broadly divided into three classes: 1) fuel vapor loss during refueling, 2) running loss or fuel loss during key-on conditions, and 3) diurnal loss, or fuel loss during key-off conditions. This invention is concerned with fuel loss while the gaseous fuel is not in use, either during key-off conditions or under key-on conditions where the gaseous fuel is not in use, such as a multi-fuel system running on another fuel (e.g. gasoline) or a hybrid system running in an electric-only mode.
The inventors herein have recognized several unique challenges specific to emission testing and leak detection for natural gas vehicles (NGVs). For example, the tests may be most accurate when the system temperatures and pressures are at steady-state equilibrium. There may be a variety of tanks or chambers where gas is trapped at a variety of pressures. Thus, neither the optimal timing for testing nor the optimal locations within the system for taking measurements are obvious for gaseous fuel systems.
In one example, the above issues may be addressed by a method, comprising: for each of a high and low pressure portion of a fuel system including a gaseous fuel, indicating degradation based on a loss of mass from the fuel system, the loss of mass based on separately tracking fuel mass in each of the portions based on respective temperatures and pressures at a first and second time instance following an engine off condition. The system may be adjusted in various ways responsive to the degradation indication, including setting a diagnostic code, displaying a message to a vehicle operator, and/or limiting engine output and/or fuel delivery to the engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.